Pumps are used in industrial and service sector applications. They can consume approximately 10-40% of electricity in these sectors. Pumping systems have potential for energy efficiency improvements. Pressure for energy efficiency improvements has led to an increasing number of variable-speed drives (VSDs) in pumping applications, because variable-speed pumping can be an effective way to reduce the total pumping costs, for example, in systems that use a wide range of flow. Pumping systems with a widely varying flow rate demand can be implemented using parallel-connected pumps. There are several control methods available for operating the parallel-connected pumps. In a simple case, parallel-connected pumps can be operated with an on-off control method, where additional parallel pumps can be started and stopped according to the desired flow rate. In systems of a more continuous flow, where precise flow regulation is used, flow adjustment can be carried out by applying throttle or speed control for a single pump, while other pumps can be controlled with the on-off method.
Compared with known rotation speed control, wherein the speed of only one pump is controlled at a time, a higher energy efficiency can be achieved if all parallel-connected pumps are speed regulated. This can be achieved if an additional parallel pump is started before the running pump reaches its nominal speed and the speeds of the parallel pumps are balanced. Starting an additional pump can increase the instantaneous power consumption of the parallel pumping system. However, using additional pumps with a lowered rotation speed can turn into an advantage if the power consumption per pumped volume (specific energy consumption) is smaller compared with a case when the same flow is delivered using only a single pump with a higher pump speed. The amount of saved energy can depend on the characteristics of the parallel pumps and the surrounding system. Realizing these potential energy savings involves advantageous starting and stopping rules for parallel-connected pumps that should be determined in the control procedure.
Energy optimization of parallel-connected, speed-regulated pumps has been studied to some extent and the results have shown that there can be an energy saving potential in the sector of parallel pumping. In order to gain energy savings, optimal speed for parallel pumps can be predicted using a mathematical-optimization-based tool suitable for programmed logic controllers. However, the suggested optimized control method uses adequate information from the system curve including start-up field measurements using pressure sensors and flow meters. On the other hand, there are applications that can determine the flow rate of each parallel pump by applying the monitoring features of the VSDs without separate flow meters. Methods that use the characteristic curves of the pumps as a model and measure pressure and/or power of the pump to determine its operating point are called model-based methods. Some model-based methods are well known in the industry. Because energy improvements in parallel pumping are welcome but sufficient initial data from continuously changing systems are often available only to a limited extent, it is justified to study if existing pumping process monitoring solutions could be used for advanced control purposes.
Because known pump control techniques can involve detailed system information, separate flow metering devices and/or start-up measurements, which may have to be repeated if the system conditions change, more versatile parallel pumping control methods are disclosed herein to, for example, improve parallel pumping processes with respect to energy efficiency, reliability or both.